The undrained shear strength and liquefaction cyclic resistance of silty sands are examined based on a large number of laboratory triaxial test results. The influence of saturation on the liquefaction triggering and occurrence of liquefaction-induced flow slides is highlighted. The laboratory triaxial tests are conducted separately in the three phases of full saturation, partial saturation and unsaturation. The past studies of the authors’ are first reviewed and some new data are added where appropriate. The different responses of silty sands in those three phases of saturation are discussed in detail.

Introduction

The liquefaction resistance and undrained shear strength of soils are the key parameters to determine the liquefaction triggering and liquefaction-induced flow slides of earth structures during earthquakes. This paper is aimed at highlighting the influence of saturation on the liquefaction resistance and undrained shear strength of silty sands, based on laboratory triaxial test results. In this paper, the phases of saturation in soils are separated into three categories, full saturation, partial saturation and unsaturation. It is known from the observation of field velocity logging tests that a velocity of propagation of primary wave of approximately 1600 m/s is observed in soil layers far below a groundwater level, indicating that these soil layers are fully saturated. However, it is also known that there is a soil layer of 3–۵ m deep immediately below a groundwater level that typically exhibits a velocity of propagation of primary wave of 500–۱۰۰۰ m/s, implying that this soil layer is partially saturated with pore water containing some minute air bubbles. On the other hand, there is a soil layer immediately above a groundwater level, where capillary water rises up through soil aggregates and a negative pore water pressure develops relative to an atmospheric air pressure, due to surface tension of pore water, leading to development of capillary suction within soil aggregates. Fig. 1 illustrates conceptually the phase transformation of saturation in soils in a diagram of pore air and pore water pressures, ua & uw, against confining stress σ, based on the work of Fredlund and Rahardjo [1]. When soils are unsaturated, though nearly saturated under an atmospheric pressure, there would be some continuous air phases within soil aggregates, where the matric suction of su = ua – uw would be present due to surface tension of pore water within soil aggregates. The surface tension tends to interact with soil aggregates and to produce bonds of soil structures, and therefore is likely to mobilise shear strength of soils.